Among display panels, such as liquid crystal display panels, reflection of external light at the surface is unfavorable. If the reflectance of external light at the surface is large, images of a viewer himself, objects that are placed around the panel, lighting devices, etc., are reflected in the panel surface, deteriorating the visibility of display. Particularly in a bright environment, excessive reflection of external images occurs. One of the existing solutions to this problem is to provide a low reflection structure at the surface of the display panel such that the reflection of external light is reduced.
As a low reflection structure which reduces the reflection at the surface of the display panel, an antireflection technique which utilizes a so-called “moth-eye”structure has been disclosed (Patent Document 1). By providing a low reflection film which has a moth-eye structure at the surface of the display panel, the reflectance of external light at the surface of the display panel is decreased to about 1% or lower. However, even if the reflectance at the surface of the display panel is decreased, reflection may occur, or the contrast ratio of display may decrease, due to light reflected inside the display panel. Therefore, it is necessary to decrease the reflectance inside the display panel.
The internal reflection of the display panel is now described with an example of a liquid crystal display panel. The liquid crystal display panel includes a pair of substrates and a liquid crystal layer interposed between the substrates. One of the pair of substrates includes pixel electrodes and TFTs that are in a matrix arrangement, and bus lines, for example. This substrate is hereinafter referred to as “TFT substrate”. The other substrate includes a counter electrode, a color filter, and a light blocking layer (black matrix). This substrate is hereinafter referred to as “counter substrate”. Typically, the counter substrate is placed at the viewer side of the liquid crystal layer. In the example liquid crystal display panel described hereinbelow, the counter substrate is placed at the viewer side of the liquid crystal layer.
External light impinging on the liquid crystal display panel is partially reflected by a surface of the liquid crystal display panel which is in contact with the air (outermost surface). In other words, external light impinging on the liquid crystal display panel is reflected at the interface between the counter substrate and the air. Usually, the counter substrate has a polarizer at the viewer side, and therefore, the reflection occurs at the interface between the surface of the polarizer and the air. In this specification, reflection by the surface of the liquid crystal display panel which is in contact with the air (hereinafter, simply referred to as “the surface of the liquid crystal display panel”) is referred to as “surface reflection”. The reflectance of this surface reflection is sometimes referred to as “surface reflectance”. Light transmitted through the surface of the liquid crystal display panel is partially reflected by the counter substrate and the TFT substrate. In this specification, reflection other than the surface reflection is referred to as “internal reflection”. The reflectance of the internal reflection is referred to as “internal reflectance”.
Among the internal reflections of the liquid crystal display panel, the internal reflection caused by the counter substrate has a large intensity. Large part of the internal reflection caused by the counter substrate is constituted of internal reflection that occurs at the interface between the transparent substrate and the light blocking layer of the counter substrate. For example, when a chromium film is used as the light blocking layer, the reflectance of the interface between the transparent substrate and the light blocking layer is about 50%. When the light blocking layer used is made of a resin in which carbon black is mixed, the reflectance of the interface between the transparent substrate and the light blocking layer is about 1.5%.
However, as described above, when a low reflection film which has a moth-eye structure is provided such that the surface reflectance is decreased to about 1.0% or lower, it is necessary to further reduce the internal reflection.
Known techniques of reducing the internal reflection caused by the light blocking layer are disclosed in, for example, Patent Documents 2 to 5.
The technique disclosed in Patent Document 2 is to provide an antireflection layer at the viewer side of the light blocking layer such that the internal reflection caused by the light blocking layer is reduced. Patent Document 2 discloses that, when chromium is used as the material for the light blocking layer and chromium oxide is used as the material for the antireflection layer, the reflectance of the light blocking layer can be decreased from 50% to 1%.
Patent Document 3 discloses that a low reflection layer, a light absorption layer, and a light blocking layer are stacked in this order, whereby the reflectance can be decreased. According to Patent Document 3, when the respective layers contain titanium oxide as a major constituent and the density of the titanium oxide decreases in order of the low reflection layer, the light absorption layer, and the light blocking layer, the internal reflectance can be decreased to 8%.
Patent Document 4 and Patent Document 5 disclose the techniques of reducing the internal reflection caused by the light blocking film by means of a low reflection layer provided between the transparent substrate and the light blocking layer. Patent Document 4 discloses that a layer which is made of a metal that contains nickel and titanium as major constituents can preferably be used as the light blocking layer and that a layer which is made of an oxide, nitride, carbide or oxynitride of a metal that contains nickel and titanium as major constituents can preferably be used as the low reflection layer. Patent Document 5 discloses that the respective layers can be formed using nickel and tantalum, instead of nickel and titanium disclosed in Patent Document 4.